This invention is directed generally to surge protectors, and more particularly, to a broadband surge protector for use in high frequency communications systems.
A surge protector is a device placed in an electrical circuit to prevent the passage of dangerous surges and spikes that could damage electronic equipment. One particularly useful application of surge protectors is in the antenna transmission and receiving systems of wireless communications systems. In such antenna systems, a surge protector is generally connected in line between a main feeder coaxial cable and a jumper coaxial cable. During normal operation of the antenna system, microwave and radio frequency signals pass through the surge protector without interruption. When a dangerous surge occurs in the antenna system, the surge protector prevents passage of the dangerous surge from one coaxial cable to the other coaxial cable by diverting the surge to ground.
One type of surge protector for antenna systems has a tee-shaped configuration including a coaxial through-section and a quarter-wave stub connected perpendicular to a middle portion of the coaxial through-section. One end of the coaxial through-section is adapted to interface with a mating connector at the end of the main feeder coaxial cable, while the other end of the coaxial through-section is adapted to interface with a mating connector at the end of the jumper coaxial cable. Both the coaxial through-section and the stub include inner and outer conductors.
At the tee-shaped junction between the stub and the coaxial through-section, the inner and outer conductors of the stub are connected to the respective inner and outer conductors of the coaxial through-section. At the other end of the stub, the inner and outer conductors of the stub are connected together creating a short. The short is indirectly connected to a grounding device, such as a grounded buss bar, by a clamp. The physical length from the junction at one end of the coaxial stub and the short at the other end of the coaxial stub is approximately equal to one-quarter of the center frequency wavelength for a desired narrow band of microwave or radio frequencies.
During normal xe2x80x9cnon-surgexe2x80x9d operation, a quarter-wave shorted stub surge protector of the above-described type permits signals within the frequency band to pass through the surge protector between the two cables connected thereto, in either direction. The direction of signal travel depends upon whether the surge protector is used on the transmission side or receiving side of an antenna system. Signals within the desired band of operating frequencies pass through one of the interfaces (depending on the direction of signal travel) to the surge protector. When passing through the surge protector, signals within the desired frequency band travel through the coaxial through-section of the surge protector.
A portion of the desired signal, however, encounters the stub while passing through the coaxial through-section. The stub scatters this signal portion which causes this signal portion to travel down the stub. After reflecting off the short-circuit, the scattered signal portion returns along the stub. Because the physical length of the stub from the junction with the inner conductor of the coaxial through-section to the short is designed to be equal to one-quarter of the center frequency wavelength for the desired band of operating frequencies, the scattered signal portion adds in phase to the non-scattered signal portion and passes through to the other end of the coaxial through-section.
When a surge occurs in the antenna system (e.g. from a lightning strike), the physical length of the stub is much shorter than one-quarter of the center frequency wavelength because the surge is at a much lower frequency than the desired band of operating frequencies. In this situation, the surge travels along the inner conductor of the coaxial through-section to the stub, through the stub to the short-circuit, through the short-circuit to the grounding attachment, and through the grounding attachment to a grounding device attached thereto. Thus, the surge is diverted to ground by the surge protector.
A drawback of the above quarter-wave stub surge protectors is that these surge protectors have a limited operating bandwidth. Original equipment manufacturers (xe2x80x9cOEMxe2x80x9d) and wireless service providers are currently required to purchase a multitude of shorted stub surge protectors to address all of the various applications that operate at different frequencies. Because there is an increasing preference towards shorted stub surge protectors because of their multiple strike capabilities and superior passive intermodulation distortion performance, an OEM or service provider would have to stock and inventory a multitude of different shorted stub surge protectors for the common allocated operating bandwidths of today""s systems (800-870 MHz, 824-896 MHz, 870-960 MHz, 1425-1535 MHz, 1700-1900 MHz, 1850-1990 MHz, 2110-2170 MHz, 2300-2485 MHz, etc.). A broadband shorted stub surge protector that can operate over this entire frequency range would allow an OEM or service provider to carry one product, obviously, simplifying inventory requirements and offering the cost advantages leveraged in higher volume purchases.
Additionally, there is a significant need for a broadband surge protector because there is an increasing amount of pressure from communities to limit the number of cell sites associated with wireless communications systems. Towards this end, there is an increasing need for wireless service providers to co-locate their operating systems employing diplexing and triplexing techniques via the existing coaxial transmission lines. This trend of multiplexing various operating frequencies has made it essential for all traditional narrowband components, such as surge protectors, to be upgraded to broadband devices.
While other types of broadband surge protectors are available being manufactured today, many employ a technique of installing a gas discharge tube between the inner and outer conductors of the coaxial surge device. While these types of devices offer broadband performance, they suffer from several undesirable features including the need for regular scheduled maintenance, the inability to withstand multiple strikes, and poor passive intermodulation distortion performance. Accordingly, there exists a need for a surge protector which has a broad operating bandwidth for use in wireless communications systems.
In the prior application of Aleksa et al., U.S. Ser. No. 09/531,398, filed Mar. 28, 2000, a broadband short-circuited stub type surge protector is described. This application is commonly owned with the present application. In the surge protector device described in the Aleksa et al. application, the stub has a hollow inner conductor which has a helical through aperture. This results in a higher impedance and a lower Q and, therefore, increased bandwidth of the shorted stub. However, the prior art short-circuited stub conductors, including the broadband conductor of the above-referenced application, act as a short to ground for low frequency and DC signals. In some applications, it is desired to pass DC through the coaxial conductor as well as the radio frequency signals. Specifically, when so-called xe2x80x9cactivexe2x80x9d antennas are utilized, it is desired to carry DC power to the antennas through the same cable as the radio frequency signals.
Briefly, active antennas are those in which electronic circuit components such as amplifiers, and the like are included on the tower closely adjacent the antenna. These electronic components require a source of DC power. In order to avoid the additional expense of running a second DC cable to provide power for these components, it is desirable to provide DC power in the same cable as the radio frequency communications signals.
However, the surge arrestors in accordance with the prior art do not permit DC and other low frequency power to pass, since they provide a short to ground for low frequencies including DC. Additionally, systems using such active antennas inject the DC current at a point towards the base or main feed, or prior to the connection to the surge protector. Unfortunately, the physical connection from the DC source of injection to the central conductor of the coaxial cable tends to interfere with the RF signals traveling through the coaxial cable, and tends to create signal distortion.